1. Field of the Invention
This invention relates generally to tension leg platforms used in the offshore oil production industry and specifically to a method and system for ballasting and de-ballasting a tension leg platform for towing, installation (lock-off to tendons) and use during in-service operation of the platform.
2. Description of the Prior Art
Tension leg platforms (TLP) are generally used offshore in deep water for the production of oil. A typical TLP has a horizontal pontoon hull structure and vertical columns supporting a platform. The hull structure provides buoyancy to the columns and platform. The TLP is anchored by tendons to pilings in the ocean floor, and it is held stationary by buoyancy-induced tension in the tendons.
The hull is generally divided into several watertight compartments in order to meet stability requirements during installation ballasting. TLPs are de-ballasted during installation to tension the tendons, maintaining the platform within design limits at all times. The de-ballasting operation is rapid to minimize the time during which the resonant frequency of TLP equals the natural period of the surrounding water. In order to rapidly de-ballast, TLPs are generally equipped with one or more pump rooms containing high-capacity pumps. However, once installation is complete, only minor in-service trim adjustments are made, so the pumps are no longer subjected high-capacity requirements.
To minimize the capital investment of permanently installed large pumps for limited use, alternative TLP designs use a single caisson in fluid communication with the ballast compartments to temporarily house a high-capacity submersible pump. Large remotely actuated valves are located low in the hull to isolate or enable flow from a particular ballast tank to the pump caisson. These valves and their associated instrumentation and controls require inspection, maintenance, repair and/or replacement, which can be costly.
A primary object of the invention is to provide a buoyant vessel with an arrangement that enables controlled ballasting and de-ballasting from the top of the hull without the need for a pump room, machinery room, valves, permanent pumps, instrumentation, wiring or controls located in the lower hull.
Another object of the invention is to provide a vessel for use as a tension leg platform which requires no access to the lower hull for machinery inspection, maintenance, repair or replacement.
Another object of the invention is to provide a method of ballasting and de-ballasting a tension leg platform for tow and installation, wherein portable submersible pumps are employed to ballast and de-ballast individual compartments having individual pump caissons.
Another object of the invention is to simplify ballast level instrumentation by providing individual compartment caissons for manual or electric soundings.
Another object of the invention is to simplify the ballast compartment vent system by providing ballast compartment vents directly to pump caissons.
The objects identified above, as well as other features and advantages of the invention are incorporated in an apparatus for ballasting and de-ballasting a tension leg platform (TLP). The TLP includes a hull which provides the buoyancy to tension the tendons and to support the topsides and four columns which support a deck. The hull includes temporary and permanent ballast tanks, but it contains no valves. The columns connecting the deck to the hull are stripped of a majority of conventional xe2x80x9cactive-columnxe2x80x9d components including electrical equipment, instrumentation, etc. Each column includes one or more internal caissons disposed in the middle of the column and which run vertically from the upper hull to the lower hull. The bottom of the caissons are connected to the bottom of permanent and temporary ballast tanks and allow deployment of submersible pumps to facilitate ballasting and de-ballasting of individual tanks. Each column also has one or more external caissons which are used to provide a source of seawater. Several submersible pumps are available for rigging into and out of the internal and external caissons and provide the ballast and de-ballast operations via an installed manifold system at the top of the columns. Venting of the ballast tanks can be accomplished through a connection to atmosphere near the top of the pump caissons. Alternatively, separate vent lines may be used to vent the ballast tanks
The invention includes a method of ballasting and de-ballasting a vessel having ballast compartments with individual pump caissons.